0x16. C - Simple Shell

C Group project Syscall

Background Context

Write a simple UNIX command interpreter.

^ “The Gates of Shell”, by Spencer Cheng, featuring Julien Barbier

Resources

Unix shell

Thompson shell

Ken Thompson

Everything you need to know to start coding your own shell

man or help: sh (Run sh as well)

Learning Objectives

• At the end of this project, you are expected to be able to explain to anyone, without the help of Google:
• Who designed and implemented the original Unix operating system
• Who wrote the first version of the UNIX shell
• Who invented the B programming language (the direct predecessor to the C programming language)
• Who is Ken Thompson
• How does a shell work
• What is a pid and a ppid
• How to manipulate the environment of the current process
• What is the difference between a function and a system call
• How to create processes
• What are the three prototypes of main
• How does the shell use the PATH to find the programs
• How to execute another program with the execve system call
• How to suspend the execution of a process until one of its children terminates
• What is EOF / “end-of-file”?

Requirements

General

• A README.md file, at the root of the folder of the project with the description of your project
• Code should use the Betty style. It will be checked using betty-style.pl and betty-doc.pl
• Shell should not have any memory leaks
• No more than 5 functions per file
• Use system calls only when you need to (why?)
• You should have an AUTHORS file at the root of your repository

GitHub

*There should be one project repository per group. Add your partner as a collaborator.

Output

Unless specified otherwise, your program must have the exact same output as sh (/bin/sh) as well as the exact same error output.

The only difference is when you print an error, the name of the program must be equivalent to your argv[0] (See below)

Example of error with sh:

$ echo "qwerty" | /bin/sh
/bin/sh: 1: qwerty: not found
$ echo "qwerty" | /bin/../bin/sh
/bin/../bin/sh: 1: qwerty: not found
$

Same error with your program hsh:

$ echo "qwerty" | ./hsh
./hsh: 1: qwerty: not found
$ echo "qwerty" | ./././hsh
./././hsh: 1: qwerty: not found
$

List of allowed functions and system calls

• access (man 2 access)
• chdir (man 2 chdir)
• close (man 2 close)
• closedir (man 3 closedir)
• execve (man 2 execve)
• exit (man 3 exit)
• _exit (man 2 _exit)
• fflush (man 3 fflush)
• fork (man 2 fork)
• free (man 3 free)
• getcwd (man 3 getcwd)
• getline (man 3 getline)
• getpid (man 2 getpid)
• isatty (man 3 isatty)
• kill (man 2 kill)
• malloc (man 3 malloc)
• open (man 2 open)
• opendir (man 3 opendir)
• perror (man 3 perror)
• read (man 2 read)
• readdir (man 3 readdir)
• signal (man 2 signal)
• stat (__xstat) (man 2 stat)
• lstat (__lxstat) (man 2 lstat)
• fstat (__fxstat) (man 2 fstat)
• strtok (man 3 strtok)
• wait (man 2 wait)
• waitpid (man 2 waitpid)
• wait3 (man 2 wait3)
• wait4 (man 2 wait4)
• write (man 2 write)

Compilation

Your shell will be compiled this way: *gcc -Wall -Werror -Wextra -pedantic -std=gnu89 .c -o hsh

Testing

Your shell should work like this in interactive mode:

$ ./hsh
($) /bin/ls
hsh main.c shell.c
($)
($) exit
$

But also in non-interactive mode:

$ echo "/bin/ls" | ./hsh
hsh main.c shell.c test_ls_2
$
$ cat test_ls_2
/bin/ls
/bin/ls
$
$ cat test_ls_2 | ./hsh
hsh main.c shell.c test_ls_2
hsh main.c shell.c test_ls_2
$

TASKS

0. betty would be proud

• Write a beautiful code that passes the Betty checks

1. Simple shell 0.1

• Write a UNIX command line interpreter

2. Simple shell 0.2

• Handle command lines with arguments

3. Simple shell 0.3

• Handle the PATH
• fork must not be called if the command doesn’t exist

4. Simple shell 0.4

• Implement the exit built-in, that exits the shell
• Usage: exit
• You don’t have to handle any argument to the built-in exit

5. Simple shell 1.0

• Implement the env built-in, that prints the current environment

6. Simple shell 0.1.1

• Write your own getline function
• Use a buffer to read many chars at once and call the least possible the read system call
• You will need to use static variables
• You are not allowed to use getline

7. Simple shell 0.2.1

• You are not allowed to use strtok

8. Simple shell 0.4.1

• handle arguments for the built-in exit
• Usage: exit status, where status is an integer used to exit the shell

9. setenv, unsetenv

• Implement the setenv and unsetenv builtin commands

setenv

• Command syntax: setenv VARIABLE VALUE

unsetenv

• Command syntax: unsetenv VARIABLE

10. cd

• Implement the builtin command cd
• Command syntax: cd [DIRECTORY]
• If no argument is given to cd the command must be interpreted like cd $HOME

11. ;

• Handle the commands separator ;

12. && and ||

• Handle the && and || shell logical operators

13. alias

• Implement the alias builtin command
• Usage: alias [name[='value'] ...]

14. Variables

• Handle variables replacement
• Handle the $? variable
• Handle the $$ variable

15. Comments

• Handle comments (#)

16. File as input

• Usage: simple_shell [filename]

AUTHORS

GitHub: Amuda Faith GitHub: Prince Edeh